Field of the Invention
The invention relates to a process for producing a titanium dioxide pigment obtainable by the sulfate process with a narrow particle size distribution, the pigment itself, and the use of said pigments in coatings and printing inks.
Technological Background of the Invention
Titanium dioxide pigment in the rutile crystal modification is usually produced by either the chloride process or the sulfate process. Rutile titanium dioxide pigment particles obtainable by the sulfate process generally have a lower hardness as compared to rutile titanium dioxide pigment particles produced by the chloride process. Therefore, the latter are more suitable for use in printing inks.
In the production by the chloride process, the raw material containing iron and titanium is reacted with chlorine, and the titanium tetrachloride formed is subsequently oxidized to titanium dioxide which is the so-called titanium dioxide base pigment. In the production by the sulfate process, the raw material containing iron and titanium is liberated in concentrated sulfuric acid in order to form titanyl sulfate. The titanyl sulfate is hydrolyzed and calcined in a rotary kiln to form titanium dioxide, the titanium dioxide base pigment. The titanium dioxide base pigment largely consists of agglomerates of primary particles, which must be deagglomerated as much as possible before further processing. The titanium dioxide base pigment obtained by the sulfate process usually has a broader particle size distribution in comparison to the titanium dioxide base pigment obtainable by the chloride process. For the deagglomeration, the titanium dioxide base pigment particles are subsequently slurried to form a suspension, and subjected to wet milling, for example, in an agitator ball mill. Subsequently, the particles are subjected to an inorganic and/or organic aftertreatment depending on the field of application.
For the general use in printing inks and coatings, in particular optical properties, such as opacity and gloss, are of a critical importance. Said properties are essentially determined by the broadness of the particle size distribution. Too fine fractions in the particle size distribution adversely affect opacity, whereas coarse particles reduce the gloss. Thus, a particle size distribution being as narrow as possible, as well as an optimized particle size are desired. The mean particle size and the broadness of the particle size distribution can be influenced by the type of wet milling. Usually, wet milling is performed by conveying the particle suspension through the agitator ball mill in one passage, which produced a relatively broad particle size distribution. In addition, however, it is known that multiple conveying through the agitator ball mill either in a cycle, the so-called circuit grinding, or in the form of several batches, the so-called multi-passage grinding, results in a narrower particle size distribution.